1. Field of the Invention
The present invention concerns a superconducting, actively shielded magnet of the type having first and second superconducting coil modules that generate a homogeneous magnetic field in a first direction in an operating volume of the magnet and the reduce the scatter magnetic field in an environment of the magnet.
2. Description of the Prior Art
In general, an actively shielded magnet is formed of two coil modules that are dimensioned and operated so that together they generate a strong magnetic field in the operating volume, and at the same time the scatter field in the environment of the magnet is significantly reduced.
Actively shielded superconducting magnets are widely used in medical magnetic resonance engineering. They are required in order to provide a strong and homogeneous magnetic field in the operating volume for polarization of specific atomic nuclei. Due to the high frequency of hydrogen in living tissue, polarized protons are excited to magnetic resonance in medical magnetic resonance engineering. After a processing of the magnetic resonance signals generated by the excited atomic nuclei, the received signals are reconstructed for an anatomical or functional imaging. Spectroscopic examinations are also implemented with the magnetic resonance technique, in which superconducting magnets are thereby likewise used for the polarization of atomic nuclei.
Cylindrical magnet designs are commonly used in medical magnetic resonance engineering. The first coil module generates a primary magnetic field in the operating volume while the second coil module generates a secondary magnetic field that strongly reduces—and, in the ideal case, should even compensated for—the scatter field generated in the environment of the magnet by the first coil module. The two coil modules are arranged coaxially, the second coil module (or shielding coil) surrounding the first coil module (or primary coil). The actual operating volume is located within the inner coil module. The two coil modules are electrically connected oppositely in series so that the magnetic fields generated by the two coil modules are aligned in opposition in the operating volume and in the environment. For example, a combination of the first coil module that generates a magnetic field of 2.25 Tesla in the operating volume, with a second coil module that generates a field of 0.75 Tesla in the operating volume, results in an effective magnetic field of 1.5 Tesla in the operating volume. Although the usable field strength in the operating volume is reduced by the active shielding, the reduction effect on the scatter field in the environment is greater. Such an actively shielded superconducting magnet is described in EP 0 144 171 A, for example.
This basic design of the actively shielded magnet has the disadvantage property that the intrinsic shielding effect against external magnetic interference fields in the operating volume is severely reduced in comparison to a superconducting magnet that is not actively shielded. The operating volume is thus no longer shielded against external magnetic interference fields.
To reestablish the shielding effect against external interference fields, additional electrically shorted shielding coils are used that are designated as an “External Interference Shield” or “EIS”. They are fashioned and arranged so that they generate a homogeneous compensation field and—in interaction with the first and second coil modules—reduce the effect of external interference fields in the operating volume and simultaneously only slightly negatively affect the homogeneous magnetic field in the operating volume. The technology of such additional EIS shielding coils is described in detail in U.S. Pat. No. 5,329,266.
The additional shielding coils, however, have only a slight shielding effect against external magnetic interference fields from interference field generators in proximity to the magnet. Such interference fields are caused, for example, by automobiles and trucks driving past, or traveling building elevators. In practice, in this cases an additional magnetic shielding (for example made of transformer plates) conventionally must be provided.